Twisted pair cables have become the physical media of choice for most local area networks. Twisted pair cables typically comprise a plurality of twisted pairs of insulated conductors surrounded by a cable jacket. The EIA/TIA 568 A Category 5 specifications (and the associated addenda) for these cables specify transmission performance requirements, such as maximum cross-talk, attenuation, etc., for transmission frequencies of up to 100 MHz.
Installed transmission systems, such as networks, may operate only at 10 Mbit/s and not use all the available bandwidth offered by cables meeting the existing specifications. Typically the Ethernet protocol used in many of the installed networks, employed only two pairs of the available four and used half-duplex transmission, i.e. one pair is transmitting while the other is receiving.
Transmission technology operating at 100 Mbit/s has been rapidly expanding in the marketplace. Also, improved cables with transmission characteristics exceeding the EIA/TIA 568 A Category 5 specifications (and the associated addenda) have also been developed. Although cables may be designed to meet current performance requirements, process variation during the manufacture of the cable may degrade cable performance to below the required specification. Furthermore, handling of the cable during installation may also degrade cable performance. For these and other reasons, cable manufacturers have developed cables with improved performance characteristics exceeding the requirements.
Newer data transmission technology has raised data rates above 1 Gigabit/s. This transmission technology and some of the existing 100 Mbit/s transmission technologies, when applied to twisted pair cables, may require the use of all four pairs in a cable in full-duplex operation (bi-directional transmission), and may require the transmission performance of the twisted pair wire cables to exceed the EIA/TIA 568 A Category 5 (and associated addenda) specifications.
For many applications, it may be desirable to minimize the delay skew or the differential in the signal velocity amongst the four pairs in order to enable fast de-scrambling of the four bit signals into a coherent bit sequence at the receiving end. In particular, four pair cables usable for bi-directional transmission may need to be high performance in order to obtain the maximum usable bandwidth. Thus, it may also be desirable to design twisted pair cables with low and uniform near and far end crosstalk, i.e. low coupling of the electromagnetic fields between twisted pairs, since crosstalk degrades cable performance. It also may be desirable to minimize the return loss (due to impedance irregularities) of the cable, since a high return loss may also impair transmission.
There are in the marketplace several cable designs that purport to meet and even exceed the Category 6 specifications. One cable design that may have gigabit capability was developed by Belden Wire & Cable Company and is disclosed in U.S. Pat. No. 5,606,151 to Siekierka et al. Siekierka et al. discloses the joining of the two insulated conductors in a pair by an adhesive or by co-extruding the two insulated conductors with a joining web. In one embodiment of the cable disclosed by Siekierka et al., each conductor is centrally disposed within an insulation. The insulations are integral with each other and are joined along their lengths by a solid integral web. Siekierka et al. discloses improved near end and far end crosstalk performance for this design embodiment of cable. The structures also are disclosed to improve the longitudinal impedance uniformity to less than +/−15 ohm and, as a result, to reduce return loss of the resulting four pair twisted cable. The observed reduction in impedance irregularities is explained by Siekierka et al. by the fact that cyclical and random irregularities that can be imparted in the twisted pair during the twisting process due to differences in twisting tension are eliminated when the conductors are first bonded together. It is also disclosed that the cable resists deformation during handling and installation.
U.S. Pat. No. 5,767,441, Brorein et al., discloses eliminating impedance variations through the pre-twisting of insulated conductors prior to twisting the insulated conductors in double twist machines or by twisting the pairs through a single twist process. The Brorein et al. process and others like it like have unleashed a flood of equipment designed to impart a back-twist to conductors of pairs in high performance cables. Brorein et al. further disclose a flat cable structure including a plurality of twisted pairs of conductors. However, the structure of these flat cable designs may pose additional transmission problems, due to inter-cable crosstalk or alien crosstalk, that is, between pairs of different cables, due to the proximity of pairs with same twist lays separated only by the jacket thickness, that may be difficult to cancel electronically through DSP filtering or other conventional techniques.
U.S. Pat. No. 5,563,377, hereinafter Arpin et al, discloses a plenum cable comprising a jacket of minimal smoke emission material surrounding a cable core comprising a plurality of twisted pair conductors. Each of the conductors of the twisted pair conductors comprises a conductor surrounded by a dual insulation, with an inner insulating layer made from a flame retardant polyolefin and an outer layer surrounding the inner layer formed from fluorinated ethylene propylene (FEP).
Other cables capable of gigabit data rates may include a central member separator to separate the individual twisted pairs from one another to reduce crosstalk, as illustrated in FIG. 1. The use of such a central separator 20 typically means that the twisted pairs 22a, b are closer to the cable jacket 24 than they would be without the central separator 20. This affects the level of alien crosstalk when two or more cables are stacked together, since the twisted pairs of adjacent cables may be closer together than they would be without the central separator. While the central separator 20 may substantially reduce crosstalk, it may not eliminate impedance irregularities. Furthermore, the insertion of a central member with the four pairs symmetrically disposed around it may be difficult to achieve and may slow down the manufacturing processes. In addition, the cable diameter may be typically increased by at least 20%. The overall cost of the cable may be also substantially increased due to the possible additional cost of the center member and higher jacketing material costs.
Another disadvantage of many prior art cables is illustrated in FIG. 2. A conventional twisted pair 26 including two conductors 28a,b respectively centered in insulations 30a,b has a figure-8 shape, which has a natural groove 32. Thus, there is a tendency for multiple twisted pairs to nest together along part of the length of the cable, as one twisted pair 34 fits naturally into the groove 32 of another twisted pair 26. This tends to increase crosstalk in the nested pairs. To attempt to prevent this nesting, conventional twisted pairs may be constructed having short twist lay lengths. However, short twist lay lengths are more difficult to achieve than long twist lay lengths, and a fairly sophisticated twisting machine may be required.